Air-coupled current transformers used in electric current measuring instruments are well known. They comprise a primary winding connected in series with a line carrying the current to be measured. The output voltage, which is matched to the instrument, is measured across a secondary winding coupled through large air gaps to the primary winding. While the output of a conventional current transformer is a voltage across a series resistor in the secondary circuit, which is in phase with the line current, the output of an air-coupled current transformer is a voltage proportional to the time derivative of the line current. Unlike conventional current transformers, air-coupled current transformers are immune to saturation effects caused by the presence of a D.C. current component on the mains.
Reference is now made to FIG. 1, wherein an air-coupled current transformer comprises a single-turn primary winding 21 with a concentric secondary coil 23. The winding and coil are contained in a five-sided magnetic box 25, which serves both as a shield and as a path for flux generated by the primary current.
Subsequent reconfigurations to accommodate dual primary windings have been made. In the design shown in FIG. 2, an air-coupled current transformer comprises two primary windings 31 and a secondary coil 33 covered by a four-sided magnetic box 35.
Immunity to external electromagnetic disturbance is achieved in the same manner in both designs. The magnetic box 25 or 35, which conducts disturbing flux around the secondary coil, provides a magnetic shield protecting against the external electromagnetic disturbance.
Thus, to provide immunity to external electromagnetic disturbance the prior art air-coupled transformers require a magnetic shield, made of a suitable nickel-iron alloy such as .mu.-metal, and require metal-forming operations such as deep-drawing or bending followed by an annealing operation, i.e., such as by annealing in dry hydrogen.
Therefore, it would be desirable to provide an air-coupled current transformer, wherein high immunity to external electromagnetic disturbance can be achieved without a magnetic shield.
Furthermore, the scale factor relating input current to output voltage depends on the size of the gap between the primary winding and the secondary coil. Due to the thermal expansion of the primary winding and the secondary coil, temperature change results in changing the size of the gap. It makes the scale factor dependent on temperature.
Thus, it also would be desirable to provide an air-coupled current transformer having scale factor independent of temperature.